1. Technical Field of the Invention
This invention is related generally to wireless communication systems, and more particularly to receiver architectures in wireless communication systems.
2. Description of Related Art
Communication systems support wireless and wire lined communications between wireless and/or wire-lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks to radio frequency identification (RFID) systems. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), wideband CMDA (WCDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution service (MMDS), RFID protocols and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, RFID device or other handheld device, communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switched telephone network (PSTN), via the Internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.) that performs analog signal processing tasks as a part of converting data to a radio frequency (RF) signal for transmission and a received RF signal to data. Most communication systems employ different RF frequency bands for transmit and receive. However, some communication systems utilize the same frequency band for transmit and receive.
For example, RFID systems typically use the same transmit and receive frequency band of operation. RFID systems generally include a reader, also known as an interrogator, and a remote tag, also known as a transponder. Each tag stores identification data for use in identifying a person, article, parcel or other object. RFID systems may use active tags that include an internal power source, such as a battery, and/or passive tags that do not contain an internal power source, but instead are remotely powered by the reader.
In general, to access the identification data stored on an RFID tag, the RFID reader generates a modulated RF interrogation signal designed to evoke a modulated RF response from a tag. The RF response from the tag includes the coded identification data stored on the RFID tag. The RFID reader decodes the coded identification data to identify the person, article, parcel or other object associated with the RFID tag. For passive tags, the RFID reader also generates an unmodulated, continuous wave (CW) signal to activate and power the tag during data transfer.
RFID systems typically employ either far-field technology, in which the distance between the reader and the tag is great compared to the wavelength of the carrier signal, or near-field technology, in which the operating distance is less than one wavelength of the carrier signal, to facilitate communication between the RFID reader and RFID tag. In near-field applications, the RFID reader and tag communicate via mutual inductance between corresponding reader and tag inductors. In far-field applications, the RFID reader generates and transmits an RF signal via an antenna to all tags within range of the antenna. The tags that receive the RF signal from the reader respond to the reader using a backscattering technique in which the tags modulate and reflect the received RF signal.
In either far-field or near-field communication schemes, the tags communicate with the reader at the same frequency transmitted by the reader. As a result, the reader has to process both the transmitted signal and the reflected signal from tags in the same frequency band. This is a challenge for single-chip transceiver RFID readers due the difference in power levels between the transmitted signal and the received signal. For example, any leakage of power from the transmitter power amplifier or reflection of transmit power off objects near the tags may appear at the input to the RFID reader receiver, which could result in desensitization in the receiver path.
Such leakage and reflections of transmit power are commonly known as “blocking signals,” in that they effectively “block” or “mask” the desired received signal. As described above, in RFID systems, the blocking signals are continuous, large and in-band, which in many instances, may prevent the RFID reader from sensing and processing the RFID tag data. In addition, in other conventional RF systems, such as WCDMA, out-of-band blocking signals may also present a problem in the receive path of single-chip transceiver architectures. Therefore, a need exists for a receiver architecture capable of canceling blocking signals in the receive path.